Re: Not as good
"Obviously, looking at Bing's use of OS maps, whilst they have a better zoom"
Bing also swap between different scaled OS maps but they also zoom to different magnifications of these so you have a choice of too small to be legible, legible and jaggies. I find the zoom painful to watch and unnecessary as only one magnification is really usable. OTOH Bing has better scrolling. If Streetmap were to disappear I suppose I'd have to use Bing but it would be an unpleasant experience.
One issue common to Streetmap and Bing is the search: they only seem able to find place names starting from the first word; if, for example, you search for Nibley their choices will include a couple of villages called Nibley, Nibley Lane, etc. but miss North Nibley. Google has the edge in this.